(1) Field of the Invention
The present invention relates generally to an apparatus and a process for synthesizing diamond, preferably single crystal diamond (SCD) by Microwave Plasma Assisted Chemical Vapor Deposition (MPCVD).
(2) Description of the Related Art
Microwave Cavity Plasma Reactors (MCPR) designs are typically able to achieve relatively high deposition rates and deposition uniformity. MCPR's have been applied to many different diamond synthesis applications. (See e.g., Zhang, J., Experimental development of microwave cavity plasma reactors for large area and high rate diamond film deposition; Kuo, K. P., Microwave assisted plasma CVD of diamond films using thermal-like plasma discharges, Ph.D. Thesis, Michigan State University, East Lansing: 1997; Ulczynski, M. J., Thesis, Michigan State University, East Lansing: 1999; Huang, W. S., Microwave plasma-assisted chemical vapor deposition of ultra-nanocrystalline diamond films, Ph.D. Thesis, Michigan State University, East Lansing: 2004; Khatami, S., Controlled synthesis of diamond films using a microwave discharge (non equilibrium plasma), Ph.D. Thesis, Michigan State University, East Lansing: 1997; Kahler, U., Microwave plasma diamond film growth, M.Sc. Thesis, Universitaet Gesamthochschule Wuppertal and Michigan State University, East Lansing and Wuppertal: 1997) 40-44] and U.S. Pat. Nos. 4,507,588, 5,311,103.) A cross sectional view of this reactor is shown in FIG. 1.
According to FIG. 1, a microwave plasma reactor creates a hemisphere-shaped plasma in close contact with the substrate and thus allows the coating of a large substrate surface. For example, using 2.45 GHz excitation, diamond films can uniformly be deposited over three to four inch diameter (80 cm2) substrates, and when excited with 915 MHz the substrate deposition diameter can be scaled to over eight inches and deposition areas can exceed 320 cm2. Additionally this reactor concept is also operationally versatile. It can be adjusted to deposit diamond over a wide range of experimental conditions; i.e. it can deposit diamond on a variety of substrates, at pressures of 0.5-200 Torr, with variable power levels of (i) 500 W-6 kW at 2.45 GHz and (ii) 3 kW-20 kW at 915 MHz.
FIG. 1 shows a cross sectional view of a typical microwave cavity plasma reactor configured for operation in the “thermally floating substrate” mode. As shown, the cavity applicator sidewall (1) is made of a cylindrical brass tube. This brass tube forms the outer conducting shell of the cavity applicator and is electrically shorted to a water-cooled baseplate assembly (2-4) and a water-cooled (21) sliding short (8) via finger stock (9). Thus the cylindrical volume bounded by the sliding short, the cavity applicator sidewall and the substrate holder/baseplate form the cavity applicator electromagnetic excitation region. Continuous wave (CW) microwave power, typically at 2.45 GHz, is coupled into the cylindrical cavity applicator through a mechanically tunable coaxial excitation probe (11). This probe is the center conductor of the coaxial waveguide (10), which, as shown in FIG. 1, is attached to the center of the sliding short (8). The movement of the sliding short changes the applicator length, Ls, while the variation of the coaxial probe (10) position adjusts the depth of penetration of the coaxial excitation probe, Lp, inside the cavity applicator. Both Ls and Lp can be independently varied up and down along the longitudinal axis of the applicator and both are adjusted: (i) to select the desired electromagnetic mode (primarily achieved by adjusting the length, Ls) and then (ii) couple microwave energy into the desired electromagnetic mode (primarily a probe depth adjustment) and (iii) to optimally match (achieved by iteratively but slightly adjusting both the probe and the short) the reactor to the input microwave system.
Transverse magnetic TM013 electromagnetic mode is used to establish the plasma discharge and is excited inside the cavity applicator by applying 2.45 GHz microwave power and length and probe tuning of the applicator to ignition conditions. For example, for an applicator with an inner diameter of 17.8 cm, the sliding short and probe lengths are adjusted to about Ls=21.7 cm and Lp=3.2 cm respectively. When approximately 20-50 W of microwave energy is coupled into the applicator and the reactor pressure is reduced to approximately 1-10 Torr, a discharge can be ignited inside the quartz dome. Then as the reactor system and discharge are brought up to the desired steady-state diamond deposition pressure, power and flow rate conditions, the applicator length, Ls, and the probe depth, Lp, are iteratively adjusted to reduce the reflected power and to achieve the desired process results such as deposition uniformity. It has been determined experimentally that the discharge loaded TM013 mode produces an axi-symmetric ellipsoid-like discharge, which is in good contact with the substrate.
The baseplate assembly consists of a water-cooled (22) and air-cooled (19) baseplate (2), an annular input gas feed plate (3) and a gas distribution plate (4). A quartz dome (5) with an inside diameter of 12.5 cm is sealed by an O-ring (20) in contact with the baseplate assembly. The thermally floating substrate holder cross section shown in FIG. 1 includes a flow pattern regulator (15), a metal tube (16), a quartz tube (17) and a holder-baseplate (6). The premixed input gases are fed into the gas inlet (23) in the baseplate assembly. The substrate (7) is placed on top of a molybdenum substrate holder (15), which is supported by the quartz tube (17). Quartz tubes of different heights may be used to change the position of the substrate with respect to the plasma to optimize (i.e. improve the uniformity, increase deposition rates, etc.) the film deposition process. The molybdenum holder (15), shown in greater detail in FIG. 2, also serves as a gas flow pattern regulator.
According to FIG. 2(a-b), a cylindrical metal tube (16) is placed concentrically inside the quartz tube. This metal tube prevents a discharge from forming underneath the substrate by reducing the electric field underneath the substrate. The metal tube (16) and the quartz tube (17) are placed on the holder baseplate (6). This holder baseplate has a 3 cm diameter hole cut in its center that allows the gases (24) to exit out of the reactor and then are pumped out of the vacuum system. The baseplate assembly (2-4), i.e. in particular the annular input gas feed plate (3), and the gas distribution plate (4), introduce an uniform ring of input gases that flow into the low pressure region inside the quartz dome. The TM013 electromagnetic fields are also focused into this region and ignite and sustain the microwave discharge (12) over the substrate (7). Air-cooling of the reactor is carried out by forcing air (or nitrogen gas) into the reactor through inlets (14) and (18), onto the dome and the interior cavity walls and then out through the screened window (13), and the optical access ports (19).
The thermally floating substrate holder configuration shown in FIG. 2(a-b) utilizes a flow pattern regulator. The objective of this regulator is to spatially control the gas flows within the quartz dome to produce a flat, uniform, disk-shaped discharge hovering over and above and in good contact with the substrate. It consists of a specially designed molybdenum substrate plate (15) with a series of holes located around its outer circumference. This holder was developed to increase the uniformity of the film deposition by changing the gas flow patterns within the reactor and especially within the discharge. The gas flows circulating within the quartz dome are directed by the flow pattern regulator to flow around and through the discharge and thereby alter the shape and position of the discharge.
Screened windows (13) are cut into the cavity wall for viewing the discharge. These windows were also used for substrate temperature measurement. A topside substrate temperature measurement can be performed by focusing the pyrometer through the window onto the substrate. When the spot size of the pyrometer is reduced to 2 mm, temperature uniformity can be measured over two to three inch diameter silicon substrates (See e.g., Kuo, K. P., An experimental study of high pressure synthesis of diamond films using a microwave cavity plasma reactor; Kuo, K. P., Microwave assisted plasma CVD of diamond films using thermal-like plasma discharges; S. S. Zuo, M. K. Yaran, T. A. Grotjohn, D. K. Reinhard, and J. Asmussen, “Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications”, Diamond and Related Materials, 17, 300-305, 2008.) Other process measurements, such as quartz dome temperature and plasma optical emission measurements also can be preformed through these screened windows.
As discussed in U.S. Pat. No. 5,311,103, several features of the Microwave Cavity Plasma Reactor are responsible for its operational versatility including: (i) independently adjustable (tunable end plate) sliding short, (ii) coupling probe, and (iii) an axially adjustable stage that supports the substrate. The sliding short and coupling probe adjustments allow the reactor to excite the discharge with the desired electromagnetic mode and also to achieve a microwave power match over a large operational (pressure and power) regime. These two adjustments together with the independent adjustment of the substrate position also enable the positioning of the discharge above and in contact with the substrate. A hemispherical or disk shaped plasma is formed over the substrate and thereby creates the conditions for large area uniform deposition and also allows a degree of substrate temperature control. In addition to smooth substrates like silicon wafers irregular shaped and multiple substrates such as inserts and tool bits can be coated.
The versatility of this reactor concept also includes the ability to specifically engineer the substrate holder configuration to a specific deposition task. Given a specific CVD deposition/synthesis application the substrate holder configuration is redesigned and modified to achieve the goals of the application. For example, when operating in the higher pressures regime (80-200 Torr) a cooling stage is added. (See e.g., FIG. 3.) Thus in this higher pressure operating regime the substrate temperature can be controlled (lowered) to be within the desirable substrate temperature diamond deposition regime; i.e. between 500-1400° C. Other examples of specially engineered substrate holders include: (i) examples given in U.S. Pat. Nos. 4,507,588, 5,311,103, 5,571,577, and 5,645,645; and the substrate assemblies designed for depositing polycrystalline diamond films on (ii) glass (See e.g. Ulczynski, M. J., Thesis, Michigan State University, East Lansing: 1999; and Ulczynski, M. J., et al., Ultra-High Nucleation Density for Diamond Film Growth at 470 and 900 C, in Advances in New Diamond Science and Technology, S. Saito, et al., Editors. 1994, Scientific Publishing Division of MVC: Tokyo); (iii) round tools as shown in FIG. 4; (iv) carbon fibers as shown in FIG. 5; (v) ring seals as shown in FIG. 6 (a-b); and the synthesis of (vi) ultrananocrystalline diamond film deposition. (See e.g., Huang, W. S., Microwave plasma-assisted chemical vapor deposition of ultra-nanocrystalline diamond films, Ph.D. Thesis, Michigan State University, East Lansing: 2004; Huang, W. S., et al., Synthesis of thick, uniform, smooth ultrananocrystalline diamond films by microwave plasma-assisted chemical vapor deposition. Diamond and Related Materials, 2006. 15(2-3): p. 341-344; Tran, D. T, Synthesis of thin and thick ultra-nanocrystalline diamond films by microwave plasma CVD, M.Sc. Thesis, Michigan State University, East Lansing: 2005; and Tran, D. T, et al., New Diamond and Frontier Carbon Technology, 2006, to be published.) In each of these example deposition applications a special substrate holder configuration was designed and was inserted into the reactor.
As previously described, when the operating pressure is increased to above about 90 Torr the substrate temperature can exceed the temperature that is allowed for diamond deposition. The substrate must then be cooled. Hence the substrate holder configuration shown in FIG. 1 must be changed from the thermally floating configuration to the cooled configuration shown in FIG. 3 and in greater detail in FIG. 7.
The water-cooled substrate holder configuration consists of a cylindrical, metallic, water-cooled stage attached to the holder-baseplate (7). As shown in FIG. 3 and FIG. 7 cooling water flows into (8) and out (9) of this stage keeping the stage at a low temperature. The molybdenum flow pattern regulator is placed on top of and is in good thermal contact with the cooling stage. A set of molybdenum insulation disks (d) are inserted between the flow pattern regulator and the substrate (10 or a) and thereby enable the variation of the substrate temperature. Gas flow patterns are similar to the gas flows shown in FIG. 2(a-b) for the thermally floating configuration. Thus the quartz tube (17) serves the same purpose for both configurations; i.e. it helps direct the gases to flow through and around the discharge, through the gas flow regulator and then exit the reactor into the vacuum system. Additional substrate temperature control is achieved by placing several disk shaped molybdenum inserts (see (d) in FIG. 7) between the substrate and the substrate cooling stage. For example, at a constant operating pressure substrate temperature variation is achieved by varying the number and thicknesses of the molybdenum disks that are placed between the substrate and the water-cooled stage. It is noted here that as the number and thicknesses of the molybdenum inserts are changed the substrate position and the sliding short positions also may have to be varied slightly in order to achieve the optimum substrate temperature and deposition uniformity. Additionally, it was determined that by varying the pressure, substrate temperatures, and input gas mixtures, i.e. CH4/H2 and N2/H2 ratios, the growth α-parameters varies, thereby adjusting the horizontal and vertical growth ratio on each diamond crystal (Jes Asmussen and D. K. Reinhard, Diamond Films Handbook, Marcel Dekker, pp. 252, 2002). Thus process optimization becomes an iterative process where the sliding short, the coupling probe, the substrate position and the substrate configuration, i.e. the number and thicknesses of the molybdenum inserts, are all adjusted for optimum deposition conditions.
Initially the microwave cavity reactor was experimentally evaluated in a variety of diamond synthesis applications over a low-pressure regime of 20-80 Torr. Using the thermally floating configuration uniform deposition was achieved over three and four-inch (80 cm2) substrates with linear, polycrystalline diamond deposition rates as high as 0.7 micron per hour. Discharge power densities were as high as 10 W/cm3. Although large area uniform deposition was achieved the low linear deposition rates were similar to the earlier experimental results, which employed the tubular reactor.
In an effort to increase the deposition rates the microwave cavity reactor was experimentally evaluated over a higher-pressure 80-160 Torr regime. In this pressure regime the microwave discharge becomes a high power density discharge. As pressure increases the discharge size decreases and the absorbed power density increases to 30-45 W/cm3. A thermal like discharge is created and neutral gas temperatures are 2500 K to over 3000 K. As described above and shown in FIG. 3, a water cooled substrate holder stage is added to enable the adjustment of the substrate temperate within the diamond deposition regime. Under these conditions, thick, two-inch diameter, uniform (better than 15%) diamond films were synthesized. For example, two-inch diameter polycrystalline disks with uniformities of 10% and with thicknesses greater than 600 microns were grown. Recently, high quality, uniform (+/−5%) polycrystalline diamond deposition over three inch diameter silicon substrates was examined. When operating in the higher-pressure regime (100-160 Torr) uniform deposition rates were as high as 4.5-7 microns per hour. These rates are eight to ten times higher than the rates at lower pressure operation. If uniformity is not a concern, i.e. if high rate deposition over smaller areas is desired then deposition rates of over 10 microns per hour are possible. Accordingly, these experiments demonstrated that by increasing the operating pressure to 100-200 Torr a high power density microwave discharge is created. This discharge produces very high radical densities, such as atomic hydrogen and carbon radicals, which are important for rapid diamond synthesis. The densities of these radical species increase with increasing pressure.
The MPCR and the associated high pressure microwave discharge can be controlled to produce high quality, uniform, thick polycrystalline films over two and three inch substrates. These experiments demonstrated that the high-pressure operation together with the associated high power density microwave discharge and the high radical species densities causes a substantial increase in polycrystalline diamond deposition rates. These results may suggest suitable commercial potential for microwave CVD diamond synthesis.
Controlled and uniform, high pressure (100-200 Torr), microwave plasma assisted CVD polycrystalline diamond synthesis was achieved by introducing a number of innovations: (i) cooling the substrate holder; (ii) the introduction of molybdenum holder inserts and the associated shaping and holding of the substrate; (iii) the reduction of the spot size for the substrate temperature measurement to about 2 mm and then during the deposition process the in-situ, online monitoring and the controlling of the substrate temperature uniformity (to less than 50 K); (iv) the adjustment of the spatial neutral gas flows within the reactor and through the discharge to improve deposition uniformity; and (v) the positioning and the controlling of the shape of the discharge so that it becomes a hemisphere-shaped plasma that hovers over and is in good contact with the substrate.
A typical process cycle includes several steps: (i) discharge ignition; (ii) pre-deposition plasma surface treatments; (iii) adjustment of the operating conditions to the desired process conditions, i.e. pressure, gas flow rates, substrate temperature and temperature uniformity, etc.; (iv) steady-state operation; (v) post-deposition plasma surface treatments; and (vi) process shut down. Discharge ignition is achieved by first adjusting the cavity applicator sliding short to a length position so that the empty cavity TM013 mode (or any TM01n) electromagnetic mode is excited within the cavity and then also by adjusting the probe depth to enable the coupling of microwave energy into the cavity applicator. For a typical 17.8 cm diameter cavity applicator the initial sliding short length for TM013 mode excitation is about 21.6 cm and the initial coupling probe depth is approximately 3.2 cm. The discharge is ignited by adjusting the pressure to about 1-20 Torr (depending on the filling gas), and then by increasing the incident microwave power and finally by coupling this power, via the adjustment of the probe and sliding short, into the applicator. After discharge ignition, the cavity applicator may have to be further adjusted by tuning the sliding short and the coupling probe so that the incident microwave energy is matched (coupled) into the cavity applicator. Once the microwave discharge is created the operating pressure is increased to the desired process conditions, which are typically between 100-200 Torr, 1.5-5 kW, and 100-800 sccm, for high-pressure operation. While increasing the pressure from a few Torr to over 100 Torr, the sliding short, coupling probe and the substrate position are adjusted to place and keep the discharge over and in good contact with the substrate. After the desired pressure and gas flow rates are reached, the probe, sliding short and the substrate position are further adjusted to achieve temperature uniformity over the substrate. Additionally the coupling probe and sliding short are tuned to achieve a suitable microwave power match while still achieving substrate temperature deposition uniformity. Thus the three independent adjustments, i.e. sliding short, coupling probe and substrate position, enable the placement of the discharge over and in contact with the substrate there by enabling uniform substrate deposition temperature and deposition uniformity.
Other microwave reactor and process technologies have been developed to produce polycrystalline diamond at high-pressure conditions. Research groups such as Element Six, Osaka University in Japan [52], Fraunhofer IAF, Freiburg, Germany [53], and the Institute of Applied Physics at Nizhniy Novgorod, Russia [54] have developed microwave assisted CVD technologies that can produce thick (1-4 mm) high quality polycrystalline windows (one to four inch diameter) for mm-wave and optical applications. (See e.g., Kobashi, K, R&D of diamond films in the Frontier Carbon Technology Project and related topics. Diamond and Related Materials, 2003. 12(3-7): p. 233-240; Funer, M., C. Wild, and P. Koidl, Novel microwave plasma reactor for diamond synthesis. Appl. Phys. Lett., 1998. 72(10): p. 1149-1151; and Vikharev, A. L., et al., Diamond films grown by millimeter wave plasma-assisted CVD reactor. Diamond and Related Materials, 2006. 15(4-8); p. 502-507.)
Recently CVD diamond synthesis for high quality, high rate homoepitaxial growth of single-crystal diamond was demonstrated. (See e.g., Yan, C. S., et al., Very high growth rate chemical vapor deposition of single-crystal diamond. Proc. Natl. Acad. Sci., 2002. 99: p. 12523-12525.) Using a microwave plasma CVD process, a MPCR (a modified Wavemat design), and synthetic high pressure high-temperature (HPHT) diamond substrates, single-crystal diamond at rates from 30-150 microns per hour were synthesized. These deposition rates were larger than the rates previously observed for polycrystalline diamond synthesis and the single-crystal diamond product was superior to the CVD synthesized polycrystalline diamond product. Thus the research results stimulated worldwide research in single crystal diamond synthesis. (See e.g., U.S. Pat. Nos. 6,858,078; 7,115,241; 6,582,513 6,858,080; 7,122,837; and 7,128,974.) While these patents generally discuss to the possibility of growing single crystal diamond using microwave plasma-assisted CVD, they do not elaborate, teach or discuss an actual or operable apparatus. Accordingly, a need still exists for effective single crystal diamond growth using microwave plasma-assisted CVD.